In automotive and various industrial applications, it is often necessary to provide a clutching/speed-trimming device that is used to connect or disconnect a prime mover to or from the rest of a powertrain and/or auxiliary devices. It is also often important to be able to continuously change the speed-ratio between the prime-mover and the driven equipment, whether they are main propulsion or powertrain devices, or auxiliary devices such as, but not limited to, cooling fans, pumps, compressors and the like. Ideally, the speed ratio can be changed while the system is running and without interrupting the torque path. These functions are particularly important, but not limited to, applications where an internal combustion engine is the prime mover. The process of connecting the torque path is typically referred to as “clutching” while the process of interrupting the torque path is typically referred to as “declutching.” The process of changing the speed ratio between the prime mover and auxiliary devices may be referred to as “speed trimming” or “speed modulation.”
Most conventional clutching devices utilize either friction (e.g. mechanical clutches) or hydraulics (e.g. fluid couplings and torque converters) in order to establish and interrupt the torque path. Magnetic clutches and couplings are gaining in popularity especially in certain industrial application but their more widespread application is limited by cost and envelope size compared to frictional and hydraulic assemblies.
During clutch engagement, the rotational speed of the driven equipment is gradually increased from zero, or another finite rotational speed lower than the rotational speed of the prime mover, to the rotational speed of the prime mover. This process of speeding up the driven equipment is referred to as “clutch modulation.” Energy from clutch modulation in traditional clutching devices is converted into heat, which is typically released into the atmosphere. Traditional clutches therefore have limited modulating capacity and/or, in the case of hydraulic systems, rather low operating efficiency. In cases where it may be necessary to provide speed trimming and/or torque modulation for extended periods of time, yet achieve high efficiency once clutched, typically two clutching devices are used: a hydraulic unit to provide for modulation, and a friction unit to achieve full clutch engagement (e.g. lock-up torque converters) once the driven equipment is sped-up.
Traditionally, clutching devices modulate the entire working torque over a speed range equal to the difference between the speed of the prime mover and the starting speed of the process machine. Therefore, clutches have a tendency of being rather large and it is difficult to achieve and automate smooth, repeatable modulation. This is especially true with friction-based clutches because coefficient of friction may change during the modulation.
There is a need for a clutching device that is smaller and more cost-effective than conventional, friction-based clutches. There is also a need for a clutching device capable of providing smooth, repeatable modulation, unrestricted in duration, and easy to automate regardless of operating conditions. Furthermore, there is a need for a clutching device capable of converting the energy of clutch modulation into an energy form that can be readily recovered and/or reintroduced into the system to provide improved efficiency of the system even when the clutch is modulating for extended periods of time.